1. Field of the Invention
The present invention relates generally to the field of heat-curable silicone resins, and more particularly to a novel method for preparing organically-modified types of such resin, which method yields a cured product that has superior physical properties.
2. Description of the Prior Art
Heat-curable silicone resins are often incorporated into coating systems, such as high temperature and maintenance paints and release coatings for culinary utensils. The heat-curable resins are used to impart the coating systems with numerous properties including heat stability, weatherability and the maintenance of gloss and adhesion. Of particular importance are the properties of hardness, impact resistance, flexibility and solvent resistance.
One such heat-curable silicone resin and a process for preparing the same are disclosed in Koerner et al., U.S. Pat. No. 4,749,764, assigned to Th. Goldschmidt AG, Germany. Koerner et al. teach a process wherein low molecular weight multifunctional alcohols are reacted with siloxanes of the general formula: ##STR1## in which OR.sup.1 is an alkoxy group of a primary or secondary aliphatic alcohol with 1 to 4 carbon atoms,
R.sup.2 is an alkyl and or phenyl group, PA1 a has a value of 1.0 to 1.5 and PA1 b has a value of 0.1 to 0.7. PA1 (a) a trialkoxysilane of the general formula EQU R.sup.2 (OR.sup.1).sub.3 Si PA1 wherein PA1 (b) a difunctional diorganosilicon compound; and PA1 (c) at least one polyol; in the presence of PA1 (d) a catalytic amount of an acidic cationic siloxane polymerization catalyst
The reaction is carried out at a temperature between about 100.degree. C. and 160.degree. C., and may incorporate the use of known transesterification catalysts.
Koerner et al. teach using multifunctional alcohols (polyols) in such a quantity that there is approximately a one-to-one correspondence between the COH groups of the polyol and the OR.sup.1 alkoxy groups of the siloxane. The COH groups of the polyol react at the OR.sup.1 sites of the alkoxy siloxane, forming an Si--O--C bond and releasing HOR.sup.1 alcohol. The HOR.sup.1 alcohol so-generated is removed until 25% to 80% of the alkoxy groups in the siloxane have been reacted.
The present inventor has investigated the physical properties of prior art cured resins, which, upon information and belief, were prepared in accordance with the teachings of Koerner et al. It was found that the cured resins exhibited some of the aforementioned physical properties to a reasonably useful level. However, it has also been found that the heat-cured compositions prepared in accordance with the present invention exhibit far superior physical properties.
A particular drawback associated with prior art resins, including those of Koerner et al., is the inability of such coatings to provide a combination of high hardness, flexibility and impact resistance. It is well-known in the field of material science that the properties of flexibility and impact resistance are generally sacrificed at the expense of hardness. In other word, hard materials often behave in a brittle manner. Resins made in accordance with the present invention, however, achieve the surprising result of exhibiting both high hardness in combination with flexibility and impact resistance.
Without limiting the present invention, it is theorized that the novel method of the invention results in a cured product having a novel chemical structure. It is believed that the method of the invention results in silicone/organic block copolymers that have a lower polydispersity and a more random structure than resins produced in accordance with the prior art. It is further theorized that the lower polydispersity and more random structure give rise to the improved physical properties associated with the cured resins of the present invention.